Although the oxides of the Groups VB and VIB elements (Cr, Mo, Nb, Ta, V, W) have frequently been used as nucleating agents in glass-ceramics, there has been little rigorous study of their effects. It has been suggested that they serve to reduce the interfacial tension between the subcritical nucleus and glass matrix, and thus encourage nucleus growth; this would cause an increase in nucleation rates in both homogeneously and heterogeneously-nucleating systems. There has been no attempt to demonstrate this, however. A test of the effects of these oxides on the steady-state nucleation rate and induction period in homogeneously-nucleating lithium disilicate glass was made by substituting one and two mole-percent of MoO₃, Nb₂O₅, Ta₂O₅, V₂O₅ or WO₃ for SiO₂, and heat-treating the glasses under identical conditions. The "two-step" method was used, consisting of nucleation for variable periods at 733 K and development at 843 K, followed by standard metallographic specimen preparation and inspection by optical reflection light microscopy. Crystal (i.e., nuclei) density was calculated using the stereological method of DeHoff and Rhines. It was shown that this method yielded results independent of crystal size for a given glass, and that the relationship between crystal size and shape supported the hypothesis that lithium disilicate nucleates as a rod-shaped particle, becoming more spherical as growth continues. It was found that MoO₃, Nb₂O₅, Ta₂O₅ and WO₃ substitutions reduced the nucleation rate in the experimental glasses and increased the induction period, the opposite of what was hypothesized. V₂O₅ substitutions yielded a slight nucleation-agent effect. The anti-nucleation agent behavior was associated with increased inhomogeneity in the heat-treated glass samples, resulting from varying cooling rates from the melt through the cross-section. Analysis of the equations linking nucleation rates and induction periods to glass properties suggests that the changes in both the nucleation parameters and the effect on the results of thermal inhomogeneity were primarily linked to higher viscosities in the substituted glasses.

No description available.

The electrical behaviour of a series of lithium silicate glasses has been characterised by the versatile a.c. impedance technique. The advantage of using a combination of complex plane and spectroscopic plots in the data analysis is shown. The compositional dependence of the conductivities of the phase separated glasses, which exhibit complex two-phase spinodal decomposition or nucleation and growth textures, is related to the volume fraction of the phases present in the conduction pathway and the tortuosity of the effective medium. The compositional dependence of the conductivities of homogeneous lithium silicate glasses is accounted for by postulating a glass structure composed of silicate anion clusters which are surrounded by a lithia-rich phase which constitutes the preferred conduction pathway. Annealing effects are also reported. The effect of surface roughness on the response of the electrode/electrolyte interface, a phenomenon originally reported by de Levie, and contact problems between the metal electrode and the glass electrolyte are discussed. Novel results regarding the effect of gold electrode recrystallisation on the a.c. response of glass electrolytes are reported. The a.c. impedance technique is shown to be a very useful, surface sensitive tool for monitoring interfacial phenomena such as atmospheric corrosion and surface ion-exchange. The technique is also successfully applied to studies of the mechanism of glass-ceramic formation, where the identification of surface crystallisation products and residual glass, by electrical measurement, is possible. Conclusive evidence for the presence of an effective medium conduction mechanism (percolation theory) in the inhomogeneous glass-ceramic, is given. The electrical data are corroborated by electron microscopy, x-ray diffractometry, energy dispersive x-ray analysis and Fourier Transform Infrared Spectroscopy.

Thesis (M.Sc.)--Victoria University of Wellington, 2008. / CD-ROM contains scattering simulation code, Mie theory code and the LabView driver to run the experiment. Includes bibliographical references.

Vitrification is a well-established waste treatment method that has been used for silicate based wastes to achieve waste volume reduction and improve chemical stability before further use. However, the poor mechanical strength of the vitrified products has led to a low commercial value and thus an unjustifiable use of energy-intensive thermal technology over the cheaper, although environmentally unsatisfactory, land disposal option. To overcome this issue, the emphasis of this work lies in demonstrating the feasibility of fabricating dense glass-ceramics from several types of waste and combination of wastes, which are generally superior to their parent glasses in their mechanical performance, as well as highly porous glassceramics to be used in building industries as an alternative to conventional waste disposal. The first part of the research work demonstrated the ease of production of an array of relatively dense glass-ceramic material from coal ash from thermal power plants using powder sintering technology. These products had robust physical and mechanical properties suitable to compete against commercially available building materials such as granite and marble for the floor and wall covering applications. A fully-dense, fine-grained, high-strength glass-ceramic was also fabricated from Feslag using an economically viable single-stage melt heat-treatment route. The ironslag derived glass-ceramic material had a unique composition, with an associated microstructure containing a high content of titanium-rich compounds, and has the potential for non-critical load-bearing applications. Finally, the thesis provides the results of an experimental study concerning with the production of highly porous glass-ceramic foams from a mixture of coal ash and waste glass with the addition of an inorganic foaming agent. The correlations between physical, thermal, and mechanical properties were carried out based on results from practical experiments, physical model studies and numerical simulations using X-ray microtomography and finite element analysis. It was concluded that, these waste-derived materials have the potential to be used in building applications where there can be a large demand to meet the large volumes of wastes available.

No description available.

No description available.

Previous attempts to produce carbon nanotube (CNT) ceramic composites have resulted in poorly dispersed, unaligned and non-continuous CNTs in the composites with modest improvements in properties. The research presented in this thesis pertains to the production of dense aluminoborosilicate (ABS) glass matrix composites containing aligned and continuous multi- walled carbon nanotubes (MWCNT) of millimetre lengths. This was achieved by infiltrating CVD grown MWCNT preforms using a precursor sol and sintering which achieved 80 ± 2% dense composites. Focused ion beam milling together with image analysis showed that the composites contained 20 ± 2 vol.% MWCNTs, which are aligned and continuous within the glass matrix. Indentation studies showed greater damage tolerance in the composite compared to unreinforced ABS glass. Under compression, there is no significant change in the compressive strength between the composite and the unreinforced glass. The bend strength of microcantilever beams were 1.4 to 1.3 GPa for the composite and glass respectively. Elastic modulus of 84 GPa and fracture toughness (Kic of up to 2.4 MPa √m were obtained for the composite. The elastic modulus and fracture toughness results are an improvement of 30 % and 240 % over that of unreinforced ABS glass. Fracture surfaces showed apparent MWCNT pullout lengths of up to ~ 1 urn. Analysis indicates that crack bridging by intact MWCNTs provides the majority of the improvement in fracture toughness. Interlayer sliding of the MWCNTs and "sword in' sheath" failure mechanism of the MWCNTs prevented the maximum potential performance, with respect to elastic modulus and fracture toughness, from being achieved. Electrical conductivity in the alignment direction of the CNTs showed improvements by a factor of 106 compared to unreinforced ABS glass. Furthermore, improvement of a factor of ~ 10 in the thermal conductivity was obtained for the composite over that of ABS glass.

Thesis (B.S.)--University of Missouri, School of Mines and Metallurgy, 1936. / The entire thesis text is included in file. Typescript. Title from title screen of thesis/dissertation PDF file (viewed July 21, 2010) Includes bibliographical references.

OBJECTIVES: To evaluate the mechanical and physical properties of three glass-ceramic CAD/CAM systems; Vita Suprinity PC (Vita ZahnFabrik), Cerrec Tessera (Dentsply-Sirona), and IPS e.max CAD (Ivoclar Vivadent). MATERIALS AND METHODS: For each material, CAD/CAM blocks were sectioned into a total of N=108 (n=12 per group) test discs with a diameter of 11.3 mm core drill and used the precision saw machine. Specimens were divided into three groups (1) glaze control, (2) polish only, and (3) ground after manufacturer glaze. Specimens were randomly assigned for the biaxial flexural strength test then each group was tested under three conditions post-treatment: (1) static, (2) thermal cycling for 5000 cycles, and (3) cyclic loading at 200,000 cycles. The biaxial flexural strength test was preformed according to ISO 6872-2015 using an Instron universal testing machine. Thermal artificial aging cycling was from 5° to 55°C, while fatigue was used for about 20% of the static failure load for 200,000 cycles. Vickers indentation was performed to test microhardness and fracture toughness. Also, surface roughness was measured for each group using a stylus contact profilometer. Additionally, surface topography and crystal microstructure analysis by SEM and crystallographic structural analysis by X-ray diffraction were done. Data were analyzed using one-way ANOVA test or least square regression (α = 0.05) and the post hoc Tukey HSD test. RESULTS: Significant differences were observed in the mean of biaxial flexural strength between the three glass-ceramics materials. The highest mean flexural strength for static was IPS e.max CAD 385.91 ± 46.23 MPa, then Vita Suprinity PC glazed 355.72 ± 72.44 MPa, followed by CEREC Tessera glazed 323.40 ± 61.01 MPa. After thermal cycling and fatiguing, there were no significant effects in flexural strength for CEREC Tessera; however, there was an increase in flexural strength for IPS e.max CAD and Vita Suprinity PC. Vickers Hardness was highest for CEREC Tessera with 7.19 ± 0.47 GPa, then Vita Suprinity PC at 7.05 ± 0.73 GPa, followed by IPS e.max CAD 5.79 ± 0.77 GPa. The fracture toughness for IPS e.max CAD was the highest with 1.97 ± 0.12 MPa.m1/2, CEREC Tessera 0.93 ± 0.05 MPa.m1/2, and Vita Suprinity PC 0.85 ± 0.08 MPa.m1/2. CONCLUSIONS: IPS e.max CAD polished outperformed Vita Suprinity PC and CEREC Tessera with respect to flexural strength under different aging conditions. In contrast, Vita Suprinity PC had the highest strength in every aging condition among the glazing group. Clinically Significant: Although all three materials might be suitable for single restorations, caution is needed when choosing material, especially as the rough surface from clinical adjustments may reduce its strength.